Anything related to hi-fi and sound reproduction from past, present, to future. Plus the humanitarian, environmental aspects of A / V consumer electronics manufacture and marketing.

Monday, August 19, 2013

EF86 Vacuum Tube: The Last Vacuum Tube?

Given that the pentode vacuum tube eventually paved the way
for the transistor, does this make the EF86 small signal pentode vacuum tube
the last vacuum tube to be developed?

By: Ringo Bones

Tracing the EF86 small signal pentode vacuum tube’s history
and development is nebulous at best, some date the tube’s release as “the late
1950s” even though Peter J. Walker released the Quad II amplifier in 1953 and
in order for him to develop his paraphase circuit / paraphrase circuit, the
EF86 tube probably was already in the market at least six months before he
released his Quad II amplifier. Despite a hazy release date, the EF86 vacuum
tube is still one of the most fascinating preamplifier pentode vacuum tubes
ever devised during the Golden Age of Stereo.

The EF86 is a high transconductance sharp cutoff pentode
vacuum tube with a Noval B9A base for audio frequency applications. It was
probably introduced a few months Peter J. Walker released his Quad II amplifier
and used the EF86 in the paraphase circuit / paraphrase circuit of the Quad II.
The EF86 was produced by Philips, Mullard, Telefunken, Valvo and GEC among
others. It is very similar electrically to the Octal base EF37A and the Rimlock
base EF40. Unlike many pentodes, the EF86 was primarily designed specifically
for audio applications. It has low noise and low microphony due to the internal
bracing and a helical / internal screen to prevent hum. With such claimed
advantages, a rubber mounted vibration resistant base was still recommended. The
EF86 has a much higher stage gain than any small signal triode vacuum tube
which makes it susceptible to microphony. This was reported to occur and worsen
with time. The pins are arranged to minimize leakage from the anode and heater
to the input grid.

The EF86 was used in many preamplifier designs during the
last decades of vacuum tube hi-fi development. And given pentode vacuum tubes
allow appreciable amounts of negative feedback to be used without triggering
instability and high-frequency rolloff due to the Miller Effect, they merely
paved the way for the development of the solid state transistor power amplifier
which used much larger amounts of negative feedback in comparison to their
vacuum tube counterparts.

Generally, pentodes or pentode type vacuum tubes generate
more noise than triodes because of the partitioning of current between the
anode and the screen grid. The EF86 has a special internal structure to
minimize this problem, allowing high gain and low noise in a single vacuum
tube. The gain available from a single EF86 will usually be in the 100 to 200
range, though going for the 200 mark may present linearity and frequency
response problems.

The main use of the EF86 vacuum tube will be in the front
ends of both preamplifiers and power amplifiers. There are some “mega” versions
of the EF86, namely the Telefunken EF804S and GECZ729 / CV4085 – both of which
are expensive. The EF86 was used as the input stage of many Mullard and GEC
circuits. I had once a “walkie-talkie” that uses EF86 vacuum tubes – three in
fact – one tube as a transmitter, one as a preamplifier for the built in
microphone and the external microphone to be connected an the third one was use
as a Wien Bridge oscillator to convert the 6-volt lead acid batteries or 4
D-cells to a higher voltage to power the tubes. Financial problems forced me to
sell it back in 2005 to a friend who once tested it as a wireless electric
guitar transmitter rig since it accepts ¼ -inch electric guitar phone plugs and
it transformed the stock sound of my friend’s 1990s era Carsbro solid state
electric guitar amplifier into something more akin to a 1965 Fender Twin.

An EF86 vacuum tube based walkie-talkie? Was this the post World War II improved version of the Motorolla SCR-536 Handie-Talkie that is usually paired with those portable radioactive thulium based X-Ray machines from the 1950s?